1. Field of the Invention
The present invention relates to a Gaussian lens for reading out color images and color originals; and, in particular, to a color-original-reading Gaussian lens which is useful for reading out images of negative and positive films at a low magnification within the range of -1/2.5.times. to -1/1.25.times..
2. Description of the Prior Art
Prevailing in recent years are developing machines known as "minilab" or "digital lab." These machines do not directly print images onto paper from films but, after reducing the film images through a lens and once capturing them onto a solid-state imaging device such as CCD so as to allow them to be subjected to various kinds of processing, print them on paper by use of laser or the like. In such machines, as their image-reading optical systems for forming images onto the solid-state imaging device, those having a high resolution are required since light-receiving devices have a high density.
On the other hand, when reading out color originals, it is necessary to obtain good images in a wide wavelength range, and it is desirable that such performances as magnification and resolution be maintained at similarly high levels among three wavelength bands of blue, green, and red. Specifically, it is necessary for the individual colors to have low chromatic aberration levels, so that imaging points from the center to peripheries align with each other with high contrast.
In general, for correcting chromatic aberration, anomalous dispersion glass materials for lenses have been in use.
As Gaussian lenses proposed for correcting chromatic aberration, those disclosed in Japanese Unexamined Patent Publication Nos. 57-108817, 62-94810, 2-124507, 4-163508, and 4-311912 have been known.
The lenses disclosed in the above-mentioned publications are either those yielding an insufficient absolute value of axial chromatic aberration correcting amount or those suitably used at a relatively high magnification of -1/10.times. to -1/5.times., thus leaving a problem in that they may not sufficiently exhibit high performances when used at a low magnification.
This problem results from the fact that the amount of occurrence of axial chromatic aberration (.DELTA.S) in a lens has a relationship of .DELTA.S=.gamma.(1-.beta.).sup.2 .multidot.f (where .gamma. is a constant) with respect to the focal length f of the lens and the imaging magnification .beta.(.beta.&lt;0) thereof, thereby becoming greater as the focal length f is longer or the absolute value .vertline..beta..vertline. of the imaging magnification .beta. is greater.